Wooden strand-based composite products, such as oriented strandboard (OSB), flakeboard, waferboard or TimberStrand, are commonly used in residential home construction. They can be formed from raw materials, such as wooden elements, such as strands and flakes. These wood products are generally manufactured in seven major stages, which may include stranding, screening, drying, blending, forming, pressing, and finishing.
Stranding is a process in which logs are cut into discrete wooden strands (also known as flakes) that typically have an aspect ratio between 1 and 20. The long axis of the strands is predominantly aligned within about 0-30° of the grain of the wood. The strand thickness can range from about 0.015″ to about 0.060″ and the length can range from about 1″ to about 15″, or even longer. In most cases, the strands are cut from round wood that has a moisture content of about 50-150%. Thus, the stranding process typically yields wet strands that must be dried prior to further processing.
Drying is usually accomplished by passing the wet strands through long rotating drums or pipes in conjunction with hot, dry air. Alternatively, strands can be dried by conveying them in a chamber with hot, dry air passing through or around the conveyance system. The drying process commonly results in strands that have a moisture content of about 1-7%. The resulting dried strands exist as a mixture of relatively large and small elements, and it is frequently desirable to screen the material in order to separate the strands on the basis of size. In many cases the smallest wooden particles, known as fines, are diverted from the product stream and are transported to a burner where they are used as fuel.
The acceptable wooden strands are then metered into large rotating drums, known as blenders, and are sprayed or otherwise mixed with bonding resin and wax. This stage is known as blending. Many blenders rotate at a rate of about 4-20 rpm and are tilted (3-8°) in order to promote material flow. A single blender can have about six or more liquid application devices distributed within it. Such application devices are frequently rotary disk atomizers, but they can also be simple spray guns. In some cases, one or two of the application devices are devoted to wax and the remaining application devices are devoted to resin. Powdered bonding resins can also be introduced into the blender. It is common for large strands and small strands to be blended separately. Strands that have been treated with bonding resin and wax are then formed into a mat. In the case of OSB and TimberStrand, treated strands are formed into a mat by dispensing them at some controlled rate through mechanical partitions that tend to align the strands in a particular orientation. As the strands pass through the alignment devices they are collected onto a continuous conveyor belt. In the case of waferboard, the strand alignment devices are not used, and the mat that collects on the conveyor belt has an essentially random orientation within the plane of the mat, which is parallel to the conveyor belt. Frequently, the strands incorporated into the top and bottom layers of an OSB mat are larger than the strands incorporated into the core (or middle) layer of the mat. In many cases the bonding resin formulas and resin application levels used in the surface layers of an OSB mat are different than those used in the core layer of the mat. In a commercial manufacturing process the mat is generally continuous in length and has a width of between about 4′ and 16′. The thickness of the mat can be in the range of about 2″ to 20″. In some cases, the continuous mat of treated strands is transported directly into a continuous hot-press, but in most cases, the mat is cut into discrete sections, which can have a length of about 8′ to 24′. These mat sections are loaded into a multi-opening hot press, which can usually press between 12 and 20 mats simultaneously. In the case of TimberStrand, the mat is loaded into a single-opening, steam-injection press.
During the process of pressing, strands are forced together and intimate contact is achieved at the strand-to-strand interfaces. Subsequent to this consolidation process, bond formation occurs as the resin undergoes curing reactions and is converted from a liquid to a load-bearing solid. The press then opens and the relatively large “jumbo” panels are ejected onto a conveyor and transported to the finishing stages of the operation. Finishing steps commonly include cutting the jumbo panels into smaller panels, such as those having dimensions of 4″×8″. Other finishing activities can include sanding, edge profiling, marking with grade stamps, grading for quality, stacking into units, sealing, labeling, strapping and packaging.
Other engineered wood-based composite products, such as medium density fiberboard (MDF), particleboard, plywood, and laminated veneer lumber, are manufactured in processes that are generally similar to that of the wooden strand-based composites.
Adhesives, or binder resins, which are commonly utilized in the blending step of the production process for wooden strand-based composites include polymeric diphenylmethane diisocyanate (pMDI), such as Huntsman's Rubinate 1840, liquid phenol/formaldehyde resole resins, such as Georgia-Pacific's 70CR66 resin; and powdered phenol/formaldehyde resole resins, such as Hexion's W3154N resin. Binders are typically applied to strands at a level of about 1-8%. In general, these adhesive types have worked well for this application, but manufacturers are constantly searching for resins that will facilitate improved distribution on the strands at minimal application rates. It is well known in the industry that binder distribution (i.e. the percentage of strand surface area covered with adhesive) can be improved by increasing the resin application rate. Indeed, increasing the resin application rate results in improved resin distribution and increased strand-to-strand bond strength. Unfortunately, high resin application rates also increase production costs and increase the rate of “build-up” or fouling of the interior surface of the blender. Conversely, decreasing the resin application rate reduces production costs and reduces the rate of “build-up” on the inside of the blender, but it also has a detrimental effect on resin distribution and strand-to-strand bond strength. When resin distribution is sufficiently poor, a significant portion of the strand population will be essentially free of adhesive on at least one major side. If two untreated strand surfaces are in direct contact in the finished board, then there will be essentially no internal bond strength at the interface between these strands. Products with many of these weak interfaces would be expected to perform poorly in a structural application. Among conventional resins, powdered phenol/formaldehyde resole resins tend to yield very good distribution at low application rates. Unfortunately, these resins are relatively expensive compared to liquid phenol/formaldehyde resins, they are “dusty”, and the application level of the powder is limited to about 3% of the strand mass, which might be insufficient to achieve a desired level of performance in the board. Higher resin loading levels can be achieved by use of pMDI or liquid phenol/formaldehyde resole resin. The extent of distribution for liquid binders is partially related to resin droplet size, which generally decreases as the resin viscosity decreases. Therefore, there is some tendency for lower viscosity liquid binders to yield improved distribution in a given application system. It is also known that there is a tendency for pMDI to absorb into a wooden strand faster and to a greater extent than that of a liquid phenol/formaldehyde resole resin. Thus, pMDI tends to yield better distribution than a liquid phenol/formaldehyde resole resin, even when they both have the same viscosity and are applied at the same level.
Accordingly, a need exists for a resin composition that can be applied to strands in a conventional blender at a given application rate wherein the resin yields improved distribution relative to that achieved with a conventional liquid resin.